From: John Woodgate
Subject: Re: wide range optical power measurement
Date: Tue, 29 Oct 2002 21:22:58 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Wed, 30 Oct 2002 07:25:39 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Jonathan Kirwan
wrote (in <4nptru40vud4gfd1d9il978oidtkld549s@4
ax.com>) about 'wide range optical power measurement', on Tue, 29 Oct
>On Tue, 29 Oct 2002 13:52:31 +0000, John Woodgate
>>I read in sci.electronics.design that Ian Buckner
>> wrote (in <firstname.lastname@example.org
>>t.com>) about 'wide range optical power measurement', on Tue, 29 Oct
>>>You may be using an LMV722, in which case if you use a resistor
>>>to get a max 4V swing at 20 uA, your output at 200pA is only 40 uV.
>>>How does this compare with the op amp offset voltage and drift?
>>>How about the bias current and drift?
>>He needs to look at a logarithmic amplifier to cover 100 dB range.
>If the application can tolerate the logarithmic errors (ripple)
>over the range, I suppose.
Ripple? Do you have progressive saturation in mind? there are other,
ripple-free, ways of making log amps.
I have a little magnetic field meter that has a 60 dB scale range on a
moving-coil meter. I suspect it uses just one silicon diode to do the
logging. (Silicon diodes give a 60 dB scale without an option.) But I'm
not sure how the temperature dependence is compensated, or even if it
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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